1. Field of the Invention
The invention relates to a method for selecting at least wireless data medium one from a number of detected wireless data media, and a read/write device for selecting the wireless data medium.
2. Description of the Related Art
Contactlessly readable Radio Frequency Identification (RFID) technology data media, also referred to as transponders or “tags”, are frequently used for the contactless identification of goods and items of all types. Due to the higher range, Ultra High Frequency (UHF) RFID systems are increasingly used instead of High Frequency (HF) RFID transponders in industrial applications, particularly in automated production plants. The disadvantage of the higher range is that a plurality of transponders are often simultaneously received in an unwanted manner, for example, also as a result of “overshoots”, which may, inter alia, be the result of reflections of the electromagnetic waves on metal surfaces. For such cases, known RFID read/write devices (often also referred to as “readers”) are provided with filter devices that are intended to ensure that only a single, specific transponder is relayed to an application, such as a control device for an automation task. Filters of this type regularly attempt to choose or select the transponder closest to the application and therefore to the read/write device from the set of received or detected transponders using receive parameters, such as via a radio signal strength.
Possible measures for increasing the reading reliability and for selecting the closest transponder in the prior art are, such as a dynamic adaptation of the transmission power, a threshold value for the expected minimum signal strength (“RSSI”) of a transponder response, or statistical evaluations via a reading frequency in the case of a repeated detection. These measures are implemented as filter criteria and filter algorithms in conventional RFID read/write devices to select the “correct” transponder, i.e., the transponder relevant to the respective application, from a broadly detected “transponder population”, which, as mentioned, should mainly involve the closest of the transponders.
The read/write devices, in particular those for the UHF RFID frequency range, are frequently used in substantially “metal” environments. For example, in industrial plants and areas with machinery and the like that have substantial areas consisting of electrically conducting material (e.g., sheet-metal cladding), overshoots often occur due to the resulting reflections of the radio signal from and to the wireless data media (transponders). This means that, in unfavorable cases, more distant data media are also detected that have a similar or sometimes even a better/higher field strength than the closer of the data media, which in most cases are preferably to be detected. Moreover, these reflections or the resulting overshoots are often dynamic, i.e., the reflection behavior in the arrangement and therefore also the overshoots can change uncontrollably due to changes in the environment, in the alignment of transponder antennas in relation to reader antennas (the “configuration”) or in other circumstances that cannot be influenced, This can already happen, for example, because an object to be identified is moved with the data medium to be identified, which is almost always the case in conventional production processes. For these reasons, the described methods for selecting the intended (usually the spatially closest) data medium which are mostly based on an evaluation of the received field strength or, in a related manner, on a variation in the employed transmission power of the read/write device, in arrangements with reflection-induced overshoots, are often not sufficiently reliable.
In short, a plurality of conventional methods exist for selecting a required or closest transponder, for example, the use of different antenna types which must, however, already be suitably selected by the user during the installation of the system, or the use of statistical evaluation algorithms, or alternatively by a “power-ramping” method, or even a combination of the described methods. The results achievable therewith still offer room for improvement, particularly in the industrial environment.
From the technical journal “ident” No. 4/2013, pp. 14 and 15, an antenna suitable for RFID use is known that can alternately use different polarizations, in particular a horizontal or vertical polarization or a right-rotating and left-rotating circular polarization. It is known from the prior art, for a detection that is as complete as possible of a number (population) of data media, to perform a plurality of detection cycles successively, in each case with a different antenna polarization, with which even data media that are unfavorably aligned in relation to the antenna or are further away can be reliably detected.
In arrangements in which the data media (RFID transponders) always have a defined alignment in relation to the antenna of the read/write device, a multipolarization antenna of this type can be permanently set to the mode that corresponds to the similarly permanent alignment of the data media (static configuration), whereby the selection of the data media based on the received field strength measurement and therefore the selection of the data media with the best received field strength in some cases shows good results, but, in arrangements with randomly aligned data media, the setting of an antenna of this type to the defined polarization type results in less satisfactory results in the selection of data media.